Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is not stored as electric charge, but is instead stored by magnetic polarization of storage elements. The storage elements are formed from two ferromagnetic layers separated by a tunneling layer. One of the two layers has at least one pinned magnetic polarization (or fixed layer) set to a particular polarity. The magnetic polarity of the other magnetic layer (or free layer) is altered to represent either a “1” (i.e., anti-parallel polarity to the fixed layer) or “0” (i.e., parallel polarity to the fixed layer). One such device having a fixed layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ is dependent on the magnetic polarity of the free layer compared to the magnetic polarity of the fixed layer. A memory device such as MRAM is built from an array of individually addressable MTJs.
Conventionally, the MTJ contacts a bottom metal through a bottom electrode. The bottom electrode contacts the bottom metal through a minimal-feature opening. The MTJ is placed on the bottom electrode in a location away from contact to the bottom metal to reduce surface roughness in the MTJ. Thus, the bottom electrode is extended larger in width than the size of the MJT. As a result, a high resistance results from increased contact resistance of the bottom electrode. Additionally, increased area overhead and bitcell size result from the extended width of the bottom electrode.
As bit cell sizes decrease, the size of the contact to the bottom metal decreases and reduces manufacturing reliability. Additionally, resistances increase as bitcell sizes decrease due to challenges in seeding and filling the bottom metal. Increased resistance results in a loss of sensitivity in conventional MTJs.
Thus, there is a need for a MTJ having a smaller bitcell size and lower resistivity.